Try this to eliminate the hum, this should be in the video's I mentioned above, but maybe not.
Using the Neutrik Speakon plug connection as you stated that you are doing, now run a single RCA interconnect from the sub red line input to a red RCA input on your preamp. This should eliminate the hum.
Using the Neutrik Speakon plug connection as you stated that you are doing, now run a single RCA interconnect from the sub red line input to a red RCA input on your preamp. This should eliminate the hum.